1. Field of the Invention
The invention is directed to an apparatus and method for rendering for display forward-looking image data.
2. Background of the Related Art
In the field of Interventional Medicine, catheters, laparoscopes, and endoscopes are frequently used to create images of tissues. These images can be used, for example, to diagnose and treat disease. Further, it may be advantageous in certain cases to employ medical images to guide and/or monitor progress during therapeutic treatment of a medical condition. Ultrasonic imaging and Optical Coherence Tomography (OCT) are imaging techniques that are well suited for such applications. For example, they can produce tomographic images in real-time from the end of, for example, a catheter, endoscope, or laparoscope. Images from these modalities tend to be displayed in planar tomographic formats. For example, when a mechanically or manually steered device is employed to rotate a transducer, the images are usually displayed as circular images. Although most medical Ultrasound and OCT images are scanned by mechanical rotation of the transducer, they can also be electronically scanned by steering the ultrasonic or optical beam from the end of the catheter or other device. New devices now incorporate manually rotated IntraVascular UltraSonic (IVUS) imaging.
When using, for example, a catheter, endoscope, or laparoscope in the confines of a vessel or body cavity for guiding and monitoring therapies, one frequently would like to visualize what lies ahead of the catheter, endoscope, or laparoscope tip. This is especially true when the therapeutic device is located on a distal most end of the catheter, endoscope, or laparoscope. Forward visualization can also be beneficial when one wishes to collect biopsy specimens from specific tissues in a minimally invasive fashion. Mechanically steered ultrasound and OCT devices are excellent at making images that are substantially perpendicular to an axis of the catheter, endoscope or laparoscope. However, for the purpose of guiding therapy, in some applications, it is preferable to have, for example, a transducer or optical fiber sweep out a conical forward-looking surface that depicts tissue that lies distal to a tip of the catheter, endoscope, or laparoscope.
The field of Interventional Medicine arose to apply less invasive technologies to the treatment of disease. For diagnostic and therapeutic guidance tools, Cardiovascular Interventionalists had fluoroscopy systems that allowed them to image the blood pool by injecting a radio-opaque contrast dye into the blood stream and then use a real time x-ray imaging technique to watch as that contrast dye passed through the vascular tree. A major limitation with this technology was that the fluoroscope did not, in fact, image the vessel tissue but rather the blood pool inside the vessel lumen. In an attempt to obtain an image of the vessel wall and not just the blood pool, ultrasonic imaging transducers were mounted at the distal end of catheters and positioned in the arteries. These transducers were typically mounted on catheters that were approximately 1 mm in diameter. The ultrasonic transducers typically had apertures of about 65 mm-0.75 mm. This approach did indeed allow the visualization of tissues that comprise the artery walls.
Optical Coherence Tomography (OCT), as described in U.S. Pat. No. 5,321,501, which is hereby incorporated by reference, is similar to ultrasonic imaging but uses reflected infrared light to create images instead of echoes from sound waves. As with ultrasound technology, it can be incorporated into an ˜1 mm diameter catheter and make images of the vessel wall in addition to the vessel lumen.
Although some mechanically steered IVUS and catheter-based OCT imaging systems create images that are a few degrees off of the perpendicular to the axis of the catheter, these systems are constructed in this way so as to minimize the reflections from the catheter sheath. The size of the reflection is greatest when it encounters the catheter sheath exactly perpendicular to the direction of propagation. Typically, these catheters image ˜+5 to −5 degrees from the perpendicular to the catheter axis. Since these images are nearly planar, they are displayed as two-dimensional images on a video monitor. However, in order to see the tissue distal to the tip of the catheter a greater angle from perpendicular must be employed. This results in a conical forward-looking surface that is swept out in front of the transducer or optical fiber. U.S. patent application Ser. No. 11/053,141, which is hereby incorporated by reference, teaches a number of methods for displaying such images using hue, saturation, and intensity of image data to give the impression of a non-planar image.
An excellent review of prior art imaging systems can be found in Bom et. al. “Early and Recent Intraluminal Ultrasound Devices”, International Journal of Cardiac Imaging, Vol. 4 1989, pp. 79-88 (hereinafter “Bom article”), which is hereby incorporated by reference. The Bom article discusses different approaches to making intraluminal images that have been tried over the years. With respect to IVUS imaging specifically, many patents exist. For example, U.S. Pat. No. 4,794,931 to Yock, which is hereby incorporated by reference, teaches both an intravascular ultrasonic imaging device, and a combined imaging and therapeutic device, and its use for guiding therapy. The catheters taught create images that are planar in nature and are intended to help guide the removal of atherosclerotic tissue by means of a circular cutting element. Continuations of this patent cover other forms of therapy guidance.
Commercially available IVUS products today create two-dimensional images that are approximately perpendicular to a longitudinal axis of the catheter. With the addition of a motorized mechanical pullback sled as discussed in U.S. Pat. No. 5,361,768, which is hereby incorporated by reference, the catheter can collect these planar images as the catheter is translated (withdrawn) from the vessel. The catheter pullback may also be done by hand. This, in effect, results in a spiral sampling of three-dimensional space that can then be displayed in various ways. Once the data has been collected, the three-dimensional volume can be re-sectioned along the longitudinal axis into what is often referred to as a Longitudinal Mode (L-Mode) image, which is just a longitudinal slice through the vessel. The primary purpose, however, of these three-dimensional IVUS images is to measure the total tissue volume of the atherosclerotic plaque. The segmentation of the imaging data into various tissue and plaque components may now be done automatically by a computer algorithm, thereby saving the operator from the tedious task of tracing the borders of the various tissue components, as discussed in, “Preintervention Lesion Remodeling Affects Operative Mechanisms of Balloon Optimized Directional Coronary Atherectomy Procedures: A Volumetric Study With Three Dimensional Intravascular Ultrasound,” Heart 2000; 83, 192-197, which is hereby incorporated by reference.
In some applications, however, it is advantageous to view the tissue in front of, or distal to, the catheter tip. Two-dimensional forward-looking IVUS imaging has been documented by a number of authors. In Evans et al., “Arterial Imaging with a New Forward-Viewing Intravascular Ultrasound Catheter, I, Initial Studies,” Circulation, vol. 89, No. 2, pp. 712-717, February 1994, which is hereby incorporated by reference, a mechanically wobbled transducer sweeps out a forward-looking, tomographic, two-dimensional image which is displayed using scan conversion techniques common in the ultrasound industry.
A companion paper authored by Ng and titled, “Arterial Imaging With a New Forward-Viewing Intravascular Ultrasound Catheter, II Three-Dimensional Reconstruction and Display of Data,” which is hereby incorporated by reference, addresses the display of the image information. By using a forward-looking transducer and rotating it on the catheter axis, a full three-dimensional conical data set is obtained. From the data set, C-Scan (two-dimensional cross-sections parallel to the transducer face) or vessel cross-sectional images are then reconstructed. After manually tracing the vessel wall borders, and spatially orienting the component two-dimensional images, they rendered a three-dimensional surface perspective of the vessel.
Another paper titled, “Three-Dimensional Forward-Viewing Intravascular Ultrasound Imaging of Human in Vitro” by Gatzoulis et al., which is hereby incorporated by reference, describes a similar mechanically wobbled transducer at the end of a catheter using post processing software to display sections in both the complete three-dimensional rendered volumes, B-Scan (two-dimensional cross-sections perpendicular to the face of the transducer) format sections, and C-scan format sections.
Further, U.S. Pat. No. 5,651,366 to Liang et al., which is hereby incorporated by reference, patented a device for making forward-looking images distal to the tip of an intravascular catheter. Not only is the diagnosis of disease envisioned, but the guidance of therapy as well. The Liang catheter employs a mirror in a distal tip of the catheter that reflects an ultrasound beam from a transducer that is in relative motion to the mirror. With the specified geometry, an approximately sector format is created to image the tissue in front of the catheter.
In a 1991 paper titled, “Investigation of a Forward-Looking IVUS Imaging Transducer” by Lee and Benekeser, which is hereby incorporated by reference, the concept of a forward-looking intravascular imaging transducer is described. In this case, the transmitted sound is reflected off a conical mirror to create a conical scan pattern in front of the catheter. No discussion of how one should accurately display the three-dimensional images is included in the manuscript. Presumably, the conical section would be displayed as a conventional IVUS two-dimensional image.
U.S. Pat. No. 6,066,096 to Smith et al., which is hereby incorporated by reference, teaches an electronically steered two-dimensional phased array transducer on the end of a catheter that can incorporate both diagnostic and therapeutic devices. This transducer, in one embodiment, is a forward-looking imaging system. This patent cites prior art that employs C-Scan displays and B-Scan displays to present any plane in the field of view.
U.S. Pat. Nos. 6,457,365 and 6,780,157, which are hereby incorporated by reference, teach a combination side-looking and forward-looking catheter based imaging system. These patents propose two techniques for displaying the forward-looking three-dimensional image data: B-Scans and C-Scans.
Optical Coherence Tomography (OCT) is analogous to ultrasonic imaging but uses light waves instead of sound waves to make images. Since the speed of light is much faster than the speed of sound, simple electronic gating techniques are not adequate for separating the reflections that emanate from the tissues at varying depths. Instead, an interferometer is used, but the rest of the signal processing is much the same as in ultrasonic imaging. Both OCT and IVUS can provide images from the distal tip of a catheter and both are able to guide interventional therapeutic procedures. The display options for intravascular ultrasonic imaging can all be readily applied to intravascular OCT imaging by one of ordinary skill in the art.
Three-dimensional rendering of both medical and non-medical images is common. Three-dimensional image rendering is a process of converting the collected image data in a manner that is amenable to human understanding, while preserving the integrity, intensity, and geometric accuracy of the image information.
In medical imaging, displays tend to fall into two general categories. The first is a surface rendering technique, where an organ surface, or a tissue surface, is detected from the image data and the surface of the tissue is displayed rather than the tissue itself. In these surface renderings, in addition to creating a three-dimensional perspective, one frequently adds lighting, shading, and/or texture to enhance the perception that one is observing a three-dimensional object.
A second technique involves simply selecting any two-dimensional slice from the three-dimensional volume and displaying that image plane on a monitor. In the case of ultrasonic imaging, these two-dimensional planes are referred to as B-Scans if they are in the plane that the sound waves traveled or C-Scans if they are perpendicular to the plane that the sound waves originally traveled. For other medical imaging modalities, they are simply referred to as cut planes or re-sampled planes.
An alternative three-dimensional display technique that allows the observer to view a three-dimensional object from the perspective of inside the object is taught in U.S. Pat. No. 5,606,454 to Williams et al., which is hereby incorporated by reference. This approach may have particular appeal for intraluminal imaging, where the structure to be displayed is in fact hollow.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.